I was replying to the post that said having mismatched plates was an oddity. I don't really think it is, it's just one way to live.
How have you come to the conclusion that others lied, not her? The sole surviving guest noted the plates to be grey. Allegedly one of the deceased (Gail?) also commented on the plates being grey before she died. Her son noted 15cm white plates when he was cleaning up after the dessert of cake and fruit. My money is on those being used for the dessert, and were likely to be considered side/bread and butter plates, not dinner plates.
Just because there were no grey plates in the home when it was searched, does not mean there weren't grey plates used for the lunch. The lunch the son was not there for, he arrived after dessert had been served. AFAIK, we have not heard how the dinner plates were cleared from the table, where they were placed, and by whom. For all we know, Erin could have been the perfect host and told her guests to stay seated, she'd clear the table, then brought the plates out into the butler's pantry and hid them somewhere odd, to be removed and discarded from the home at a later date.
Have we clarified anything more than "grey", "white" or "15cm"? To me, if the plates were a very pale grey, they may appear white, and a "dirty white" may appear light grey. Has there been any clarification on what type of grey they were?